1. Field of the Invention
The disclosed technology relates to a micromirror device and in particular to a method for operating such a micromirror device.
2. Description of the Related Technology
Micromirrors are microelectromechanical systems (MEMS) that can be used in several applications, ranging from scanning mirrors (optical scanning, optical switching) to projection displays.
For example, the digital micromirror device (DMD), described by L. J. Hornbeck in “Digital Light Processing and MEMS: Timely Convergence for a Bright Future”, Proc. SPIE, Vol. 2639, p. 2, 1995, comprises a micromirror array used as a spatial light modulator (SLM) in projection displays. The DMD comprises an array of light switches that use electrostatically controlled MEMS mirrors to modulate light digitally, thereby producing images on a screen.
The mirrors, with a one-to-one relationship to the pixels of the display, are arranged in a rectangular array. They can rotate between two extreme positions depending on the state of an underlying memory cell, and thus reflect incoming light into a lens (ON state) or not into the lens (OFF state).
The ON state corresponds to a pixel on the screen that is illuminated (“white” pixel) and the OFF state corresponds to a dark pixel (“black” pixel) on the screen.
For producing the sensation of grayscale to the observer's eye, binary pulse width modulation (PWM) is used. Video frames are divided into n sub-frames. During every sub-frame, a mirror is either in the ON state (white) or in the OFF state (black). Assuming a light source with constant intensity, the ratio of ON and OFF states within a frame then determines the gray level of the pixel for that frame.
Using this method, the number and the distribution of gray levels depends on the number of binary sub-frames or bitplanes. With n sub-frames or bitplanes this method gives rise to (n+1) linear gray levels. Digital Pulse width modulation may lead to severe speed requirements (data transfer rates) for the on-chip electronics and for complete elimination of contouring effects.
In U.S. Pat. No. 6,466,358 an analog pulse width modulation (PWM) method is described that can be used for addressing a digital micromirror array. This method solves some of the problems related to binary pulse width modulation, such as the high cost in terms of data transfer rates and the hardware needed to sample and process the image data.
In the method described in U.S. Pat. No. 6,466,358, the voltage signal applied to the micromirror addressing electrodes results from a comparison between analog input signals. This comparison is done by means of a transistor circuit in the CMOS layer, i.e. this analog PWM occurs at the electronic level. For each pixel, there is a need for at least six transistors that can withstand large voltages, leading to relatively large chip area consumption.
Furthermore, as the method is based on a comparison between analog voltages, switching of a micromirror depends on a transistor threshold voltage. It may be difficult to control this threshold voltage accurately, and furthermore the threshold voltage may vary on a chip and thus it may be different from pixel to pixel. This may cause fixed pattern noise.
U.S. Pat. No. 5,583,688 discloses a digital micromirror device, wherein a mirror is supported by a center support post attached to two torsion hinges by a landing hinge yoke. The ends of the torsion hinges are attached to two support posts which hold the hinges above the substrate and allow the hinges to twist in a torsional fashion.
US2006/109539 discloses a method for driving an optical deflecting device array, which arranges a plurality of optical deflecting mirrors. In the disclosed method, the mirror position is controlled by several sets of electrodes, each electrode corresponding to one possible mirror position.